U.S. patent application number 16/616201 was filed with the patent office on 2020-03-19 for lithium metal secondary battery and manufacturing method therefor.
This patent application is currently assigned to LG Chem, Ltd.. The applicant listed for this patent is LG Chem, Ltd.. Invention is credited to Baeck-Boem Choi, Min-Wook Kim, Cha-Hun Ku, Sang-Kyun Lee.
Application Number | 20200091550 16/616201 |
Document ID | / |
Family ID | 67225184 |
Filed Date | 2020-03-19 |
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United States Patent
Application |
20200091550 |
Kind Code |
A1 |
Choi; Baeck-Boem ; et
al. |
March 19, 2020 |
Lithium Metal Secondary Battery and Manufacturing Method
Therefor
Abstract
Provided is a lithium metal secondary battery ensuring
electrode-separator adhesive strength and a method for fabricating
the same. The lithium metal secondary battery according to the
present disclosure includes a negative electrode, a separator and a
positive electrode, the negative electrode including a lithium
metal foil as a negative electrode material, wherein a nano imprint
pattern structure is formed on a lithium metal foil surface which
is a surface of the negative electrode facing the separator, and
the negative electrode and the separator are adhered to each
other.
Inventors: |
Choi; Baeck-Boem; (Daejeon,
KR) ; Ku; Cha-Hun; (Daejeon, KR) ; Kim;
Min-Wook; (Daejeon, KR) ; Lee; Sang-Kyun;
(Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG Chem, Ltd. |
Seoul |
|
KR |
|
|
Assignee: |
LG Chem, Ltd.
Seoul
KR
|
Family ID: |
67225184 |
Appl. No.: |
16/616201 |
Filed: |
December 21, 2018 |
PCT Filed: |
December 21, 2018 |
PCT NO: |
PCT/KR2018/016540 |
371 Date: |
November 22, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2004/027 20130101;
H01M 10/052 20130101; H01M 10/0585 20130101; B82Y 40/00 20130101;
H01M 10/0525 20130101; H01M 4/1395 20130101; H01M 4/382 20130101;
H01M 10/0468 20130101; H01M 2/168 20130101; H01M 4/043
20130101 |
International
Class: |
H01M 10/0525 20060101
H01M010/0525; H01M 4/38 20060101 H01M004/38; H01M 10/04 20060101
H01M010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2017 |
KR |
10-2017-0180546 |
Dec 17, 2018 |
KR |
10-2018-0163554 |
Claims
1. A lithium metal secondary battery, comprising: a negative
electrode, a separator and a positive electrode, the negative
electrode including a lithium metal foil as a negative electrode
material, wherein the lithium metal foil comprises a nano imprint
pattern structure is on a lithium metal foil surface which is a
surface of the negative electrode facing the separator, and the
negative electrode and the separator are adhered to each other.
2. The lithium metal secondary battery according to claim 1,
wherein the separator is filled in the pattern structure such that
there is a physical bond between the negative electrode and the
separator.
3. The lithium metal secondary battery according to claim 2,
wherein the physical bond is a result of the separator being filled
in the pattern structure by deformation.
4. The lithium metal secondary battery according to claim 2,
wherein the physical bond is a result of a separator binder applied
to a surface of the separator being filled in the pattern
structure.
5. A method for fabricating a lithium metal secondary battery,
comprising: stacking and laminating a negative electrode, a
separator and a positive electrode, the negative electrode
including a lithium metal foil for a negative electrode material,
wherein a nano imprint pattern structure is formed on a lithium
metal foil surface which is a surface of the negative electrode
facing the separator; and adhering the negative electrode and the
separator.
6. The method for fabricating a lithium metal secondary battery
according to claim 5, wherein in the adhering, the separator is
filled in the pattern structure to form a physical bond between the
negative electrode and the separator.
7. The method for fabricating a lithium metal secondary battery
according to claim 5, wherein: adhesion of the negative electrode
and the separator is formed in the lamination, or adhesion of the
negative electrode and the separator is formed by first laminating
the negative electrode and the separator to manufacture a negative
electrode-separator adhesion structure, or adhesion of the negative
electrode and the separator is formed by manufacturing the negative
electrode-separator adhesion structure, then laminating the
positive electrode, and a lamination load is 10 kgf.
8. The method for fabricating a lithium metal secondary battery
according to claim 6, wherein the physical bond is formed when the
separator is filled in the pattern structure by deformation.
9. The method for fabricating a lithium metal secondary battery
according to claim 6, wherein the physical bond is formed when a
separator binder applied to a surface of the separator is filled in
the pattern structure.
10. The method for fabricating a lithium metal secondary battery
according to claim 5, wherein forming the pattern structure
comprises directly applying pressure to the lithium metal foil
surface using a pattern mold.
11. The method for fabricating a lithium metal secondary battery
according to claim 5, wherein a distance between patterns in the
pattern structure is not greater than 1.60 .mu.m.
12. The lithium metal secondary battery according to claim 1,
wherein the lithium metal foil has a thickness within the range of
20 .mu.m to 40 .mu.m, and the nano imprint pattern structure has a
height of 50 nm to 500 nm.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a lithium metal secondary
battery using a lithium (Li) metal as a negative electrode material
without a separate negative electrode active material and a method
for fabricating the same. The present application claims priority
to Korean Patent Application No. 10-2017-0180546 filed in the
Republic of Korea on Dec. 27, 2017 and Korean Patent Application
No. 10-2018-0163554 filed in the Republic of Korea on Dec. 17,
2018, the disclosure of which is incorporated herein by
reference.
BACKGROUND ART
[0002] Secondary batteries can be recharged repeatedly, and they
are gaining attention as an alternative to fuel energy. They have
been primarily used in traditional handheld devices such as mobile
phones, video cameras and electric power tools. Recently, the range
of applications tends to gradually extend to electric vehicles
(EVs, HEVs, PHEVs), large-capacity energy storage systems (ESSs)
and uninterruptible power systems (UPSs).
[0003] A secondary battery includes an electrode assembly including
a positive electrode, a negative electrode and a separator
interposed between, and an electrolyte that electrochemically
reacts with active materials coated on the positive electrode and
the negative electrode, and a typical secondary battery is a
lithium ion secondary battery in which electrochemical reactions
occur in the positive electrode and the negative electrode by the
action of lithium ions as working ions during charging and
discharging. The conventional lithium ion secondary battery applies
lamination in the assembly process to achieve the adhesive strength
between the electrode and the separator within the electrode
assembly. The lamination is a process that thermally joins the
separator and the electrode. The lamination adheres the separator
and the electrode stacked one on the other by heat, and as a
result, increases the adhesive strength between the separator and
the electrode. The rough surface shape of the conventional
electrode including an active material, a conductive material and a
binder makes it easy to form an electrode-separator adhesion
through lamination with the separator.
[0004] Recently, in an effort to improve the energy density of
lithium ion secondary batteries, a great attention is paid to the
need for development of next-generation secondary batteries
directly using a lithium metal foil as a negative electrode
material without a separate negative electrode active material. A
lithium metal has a high ionization tendency and low density as
well as very low standard electrode potential and very high
specific capacity. Although a lithium metal has problems such as
the internal short of a battery caused by lithium dendrite growth
and a risk that explosion may occur due to the exposure to
moisture, if the problems are solved, the highest energy density
can be achieved, and because of this advantage, a lithium metal is
worth further research.
[0005] However, when a lithium metal foil with a flat and smooth
surface is used as the negative electrode and forms an adhesive
interface with the separator, it is impossible to expect a physical
adhesion by the shape deformation of (the binder in) the separator
that has been achieved through the rough active material surface
shape of the conventional lithium ion secondary battery, and only a
chemical adhesion by electrostatic attraction might be relied on.
Accordingly, a lower adhesion than the electrode-separator adhesive
strength achieved in the conventional lithium ion secondary battery
may be formed in the assembly process of lithium-sulfur batteries,
lithium-air batteries and all solid state batteries to which a
lithium metal foil may be applied. This limits the assembly
processability of next-generation secondary batteries, causing
defects such as separation and meander tolerance.
[0006] FIG. 1 is a diagram illustrating a problem when a lithium
metal foil with a flat and smooth surface is used as a negative
electrode and adhered to a separator.
[0007] As shown in (a) of FIG. 1, assume that a lithium metal foil
negative electrode 1 with a flat and smooth surface, a separator 2
and a positive electrode 3 are stacked and laminated to form a
monocell 4 as shown in (b). Generally, the positive electrode 3
including a PVDF based binder and an active material of metal oxide
having a high elastic modulus forms a stronger interfacial adhesion
with the separator 2 than the lithium metal foil negative electrode
1 with a flat and smooth surface. By this reason, a defect such as
bending occurs in the monocell 4 due to an adhesive strength
difference between negative and positive electrodes as shown in
(c). In case that there is a great difference in the extent of
adhesion with the separator between the negative electrode and the
positive electrode, if the monocell 4 bends too much due to a
property difference between negative and positive electrodes after
lamination, there is a very high likelihood that the lithium metal
foil negative electrode 1 with a flat and smooth surface having a
low adhesive strength will be separated as shown in (d).
[0008] Meanwhile, in the case of all solid state batteries, in some
cases, the lamination pressure is applied to achieve adhesion of
electrode-separator (electrolyte layer) of a unit cell. Due to the
stiff (high elasticity) active material, the soft separator
(electrolyte layer) may be partially damaged, causing a short. To
solve this problem, Patent Literature 1 proposes the design of the
electrode having a lower active material composition toward the
interface of the separator (electrolyte layer) to prevent
electrical shorting of the separator (electrolyte layer) even
though strong lamination pressure is applied. However, this
approach is difficult to technically implement and has low economic
efficiency, and besides, cannot be applied to lithium metal all
solid state batteries using no negative electrode active
material.
RELATED LITERATURES
Patent Literatures
[0009] (Patent Literature 1) JP2011-124028 A
DISCLOSURE
Technical Problem
[0010] The present disclosure is directed to providing a lithium
metal secondary battery ensuring electrode-separator adhesive
strength.
[0011] The present disclosure is further directed to providing a
method for fabricating a lithium metal secondary battery ensuring
electrode-separator adhesive strength.
[0012] 20
Technical Solution
[0013] To achieve the above-described object, a lithium metal
secondary battery according to the present disclosure includes a
negative electrode, a separator and a positive electrode, the
negative electrode including a lithium metal foil as a negative
electrode material, wherein a nano imprint pattern structure is
formed on a lithium metal foil surface which is a surface of the
negative electrode facing the separator, and the negative electrode
and the separator are adhered to each other. Here, preferably, the
separator is filled in the pattern structure to form a physical
bond between the negative electrode and the separator.
[0014] In this instance, the physical bond may be formed when the
separator is filled in the pattern structure by deformation, and
may be formed when a separator binder applied to a surface of the
separator is filled in the pattern structure. Preferably, the
lithium metal foil is 20.about.40 .mu.m thick, and the surface
pattern structure is 50.about.500 nm high.
[0015] Preferably, an adhesive strength between the negative
electrode and the separator may be 3 times or more than that of the
conventional art under a same lamination load used.
[0016] To achieve another object, a method for fabricating a
lithium metal secondary battery according to the present disclosure
includes stacking and laminating a negative electrode, a separator
and a positive electrode, the negative electrode including a
lithium metal foil as a negative electrode material, wherein a nano
imprint pattern structure is formed on a lithium metal foil surface
which is a surface of the negative electrode facing the separator,
and adhering the negative electrode and the separator. Adhesion of
the negative electrode and the separator may be formed in the
lamination, or may be formed by first laminating the negative
electrode and the separator to manufacture a negative
electrode-separator adhesion structure, or manufacturing the
negative electrode-separator adhesion structure, then laminating
the positive electrode, and a lamination load may be 10 kgf.
[0017] Forming the pattern structure includes directly applying the
pressure to the lithium metal foil surface using a pattern mold.
Preferably, the pattern mold may form a nano pattern having a
height of 50.about.500 nm. Accordingly, preferably, an organic mold
manufactured by replicating the pattern mold manufactured by a
bottom-up method such as self assembly is used.
[0018] Preferably, a distance between patterns in the pattern
structure is not greater than 1.60 .mu.m.
[0019] The lithium metal secondary battery according to the present
disclosure may have, as a unit cell, a monocell and a bicell
manufactured through the lamination, and may be implemented as a
stack cell by stacking the unit cells, folding the unit cells in
zigzag, and stacking and folding the unit cells.
Advantageous Effects
[0020] According to the present disclosure, when the negative
electrode includes a lithium metal foil as the negative electrode
material, a physical bond is formed between the negative electrode
and the separator by shape deformation of the separator, ensuring a
close adhesion between the negative electrode and the separator.
With the improved interfacial adhesion between the negative
electrode and the separator, it is possible to avoid stress that
may occur in the negative/positive electrode, thereby preventing
the bending of the unit cell and the negative electrode separation.
The lithium metal foil having the surface pattern structure forms a
strong adhesion with the separator by physical adhesion in the
lamination process, thereby improving the assembly
processability.
[0021] The lithium metal secondary battery according to the present
disclosure may have a monocell and a bicell as the unit cell, and
may be implemented by stacking the unit cells, folding the unit
cells in zigzag, and stacking and folding the unit cells. It is
possible to fabricate various types of secondary batteries
irrespective of the type of unit cell, and the improved negative
electrode-separator adhesive strength leads to significant
improvement in the properties of both the unit cell and the stack
cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The accompanying drawings illustrate the embodiments of the
present disclosure, and together with the following detailed
description, serve to provide a further understanding of the
technical aspects of the present disclosure, and thus, the present
disclosure should not be construed as limited to the drawings.
[0023] FIG. 1 is a diagram illustrating a problem when a lithium
metal foil with a flat and smooth surface is used as a negative
electrode and adhered to a separator.
[0024] FIG. 2 shows a lithium metal foil negative
electrode-separator adhesion structure included in a lithium metal
secondary battery according to the present disclosure.
[0025] FIG. 3 shows another example of a lithium metal foil
negative electrode-separator adhesion structure included in a
lithium metal secondary battery according to the present
disclosure.
[0026] FIG. 4 is a diagram illustrating an improvement effect when
a lithium metal foil having a surface pattern structure is used as
a negative electrode and adhered to a separator according to the
present disclosure.
[0027] FIG. 5 is a photographic image of a process of preparing an
experimental example sample according to the present
disclosure.
[0028] FIG. 6 is a cross-sectional view of DVD-R used for nano
imprint.
[0029] FIG. 7 is a photographic image of a general lithium metal
foil with a flat and smooth surface as a comparative example.
[0030] FIG. 8 is a graph showing 90.degree. peel-off test results
of an experimental example of the present disclosure and a
comparative example.
[0031] FIG. 9 is a graph showing 90.degree. peel-off test results
of another experimental example of the present disclosure and a
comparative example.
[0032] FIG. 10 is a flowchart showing a method for fabricating a
lithium metal secondary battery according to an embodiment of the
present disclosure.
[0033] FIG. 11 is a diagram illustrating a lithium metal secondary
battery according to another embodiment of the present
disclosure.
[0034] FIG. 12 is a diagram illustrating a lithium metal secondary
battery according to still another embodiment of the present
disclosure.
MODE FOR CARRYING OUT THE INVENTION
[0035] Hereinafter, the embodiments of the present disclosure will
be described with reference to the accompanying drawings. Prior to
the description, it should be understood that the terms or words
used in the specification and the appended claims should not be
construed as limited to general and dictionary meanings, but
interpreted based on the meanings and concepts corresponding to the
technical aspects of the present disclosure on the basis of the
principle that the inventor is allowed to define the terms
appropriately for the best explanation. Therefore, the embodiments
described herein and illustrations shown in the drawings are just
an embodiment of the present disclosure, but not intended to fully
describe the technical aspects of the present disclosure, so it
should be understood that a variety of other equivalents and
modifications could be made thereto at the time the invention was
made.
[0036] In the embodiment described below, it should be interpreted
as that even though the name changes depending on the type of
electrolyte or separator used in a lithium metal secondary battery,
the type of packaging used to package the lithium metal secondary
battery and the internal or external structure of the lithium metal
secondary battery, the lithium metal secondary battery covers any
battery using a lithium ion as a working ion and including a
lithium metal foil as a negative electrode material.
[0037] Additionally, the lithium metal secondary battery is not
limited to the number of components. Accordingly, the lithium metal
secondary battery should be interpreted as including a unit cell
including an assembly of positive electrode/separator/negative
electrode and an electrolyte in a packaging material as well as an
assembly of unit cells, a module including assemblies connected in
series and/or in parallel, a pack including modules connected in
series and/or in parallel, and a battery system including packs
connected in series and/or in parallel.
[0038] The present disclosure proposes electrode-separator adhesion
improvement using a surface pattern structure of the lithium metal
foil. The nano imprint technique is used to form the surface
pattern structure on the lithium metal foil. The surface pattern
structure is formed by directly applying the pressure to the
lithium metal foil surface using a pattern mold. The lithium metal
foil having the surface pattern structure forms an adhesion with
the separator by physical adhesion in the lamination process,
thereby improving the assembly process ability.
[0039] When the lithium metal foil is used as the negative
electrode material, a lower interfacial adhesion with the separator
may be formed than the positive electrode using a positive
electrode active material. To solve this problem, after extensive
studies for improving adhesion by applying a separator binder to
the lithium metal foil surface or the separator surface, performing
corona, RIE and acid treatment on the lithium metal foil surface or
the separator surface to improve the electric charge properties and
designing the surface roughness of the lithium metal foil surface
or the separator surface to improve physical adhesion (anchoring),
the inventors verify the effect of the nano imprint pattern
structure formed on the lithium metal foil surface and propose the
present disclosure.
[0040] The present disclosure relates to a lithium metal secondary
battery. The lithium metal secondary battery of the present
disclosure includes a lithium metal foil as a negative electrode
material, and is the same as a general lithium metal secondary
battery in terms of including a negative electrode having a
negative electrode material, a separator and a positive electrode
and its fabrication method. However, the lithium metal secondary
battery of the present disclosure and the conventional lithium
metal secondary battery have different surface shapes of the
negative electrode material, and lamination is different from that
of the conventional method for fabricating a lithium metal
secondary battery.
[0041] FIG. 2 shows the lithium metal foil negative
electrode-separator adhesion structure included in the lithium
metal secondary battery according to the present disclosure.
FIG.
[0042] 3 shows another example of the lithium metal foil negative
electrode-separator adhesion structure included in the lithium
metal secondary battery according to the present disclosure. First,
as shown in (a) of FIG. 2, a lithium metal foil negative electrode
10 having a surface pattern structure 12 and a separator 20 are
prepared and laminated to manufacture a lithium metal foil negative
electrode-separator adhesion structure 22 as shown in (b), and a
positive electrode is laminated together to manufacture an
electrode assembly which is then put in a packaging material such
as a pouch case, followed by electrolyte solution injection and
sealing to manufacture a lithium metal secondary battery. The
lithium metal secondary battery has the pattern structure on the
lithium metal foil surface that is a surface of the negative
electrode 10 facing the separator 20, and has adhesion between the
negative electrode 10 and the separator 20.
[0043] The nano imprint technique is applied to form the surface
pattern structure 12 on the lithium metal foil. The surface pattern
structure 12 is formed by directly applying the pressure to the
lithium metal foil surface using a pattern mold. When the separator
20 is adhered to the lithium metal foil negative electrode 10
having the surface pattern structure 12, the separator 20 is filled
into the surface pattern structure 12 by shape deformation.
Accordingly, adhesion is formed between the negative electrode 10
and the separator 20 by physical bond in the lamination process,
thereby improving the assembly processability.
[0044] In this instance, the physical bond may be formed when the
separator 20 is filled into the surface pattern structure 12 by
deformation, and as shown in FIG. 3, the physical bond may be
formed when a separator binder layer 18 applied to the separator 20
surface is filled into the surface pattern structure 12. In this
instance, the separator binder layer 18 may be filled into the
surface pattern structure 12 in part, or may form a conformal layer
on the surface of the surface pattern structure 12, or may be only
present on the highest protrusion area of the surface pattern
structure 12.
[0045] Preferably, there is no gap between the separator 20 and the
lithium metal foil negative electrode 10, and when the separator
binder layer 18 exists, there is no gap between the separator 20,
the separator binder layer 18 and the lithium metal foil negative
electrode 10.
[0046] The separator 20 may be a polyolefin based polymer such as
PE and PP, and the separator binder layer 18 may be a PVDF based
binder additionally formed and provided on the separator 20.
[0047] As described below, a method for forming the surface pattern
on the lithium metal foil negative electrode 10 may be an imprint
method using the pressure, such as nano imprint. In this instance,
the lithium metal is pressed down when subjected to the pressure,
and the modulus and density of the material increase and the
lithium metal becomes rigid. When a relatively soft separator
binder layer 18 contrary to the rigid lithium metal is further
included, a better adhesive interface may be formed. Referring back
to FIG. 2, when the lithium metal foil is used as the negative
electrode material, the present disclosure uses the lithium metal
foil roll-pressed to the thickness d of 100 .mu.m or less to
achieve the energy density of the battery. The thickness d is
calculated by an average distance between two outermost surfaces of
the lithium metal foil. The lithium metal foil used as the negative
electrode material may or may not include a current collector. When
the lithium metal foil includes a current collector, the lithium
metal may be formed, for example, 10.about.100 .mu.m thick on two
surfaces of 5.about.20 .mu.m thick copper foil current collector.
When the lithium metal foil does not include a current collector,
the lithium metal foil alone may be roll-pressed to 100 .mu.m or
less without a separate current collector. The height h of the
surface pattern structure 12 is 1 .mu.m or less. The height h
corresponds to the height of a protrusion or the depth of a groove,
and is calculated by a height average of the extent to which the
surface pattern structure 12 protrudes or recedes from the lithium
metal foil. The thickness of the lithium metal foil more than 100
.mu.m is unstable for achieving high energy density due to the
increases in thickness and volume in the manufacture of the stack
cell. Preferably, the thickness ranges 20.about.40 .mu.m. In this
case, the height h of the surface pattern structure 12 is more
preferably 50.about.1,000 nm. The thickness of the lithium metal
foil less than 20 .mu.m is difficult to handle and is problematic
for structural rigidity. When the metal current collector (for
example, a copper foil) is applied, the electrode may be
manufactured by the roll-pressing adhesion of different types of
metals between the lithium and the current collector, but when the
lithium metal foil thinner than 20 .mu.m is applied, there is
concern about damage to the lithium metal when roll-pressing.
Additionally, when the battery is manufactured using the lithium
metal foil as the negative electrode, in the event of reversible
thickness changes of the lithium metal during charging and
discharging while in real use, if the thickness of the lithium
metal is very small, the structural stability is not ensured. The
thickness of the lithium metal foil more than 40 .mu.m is
undesirable from the viewpoint of energy density due to the
increases in thickness and volume in the manufacture of the stack
cell. The height of the surface pattern structure less than 50 nm
is insufficient for a sufficient physical bond between the negative
electrode and the separator. It is difficult to expect that the
lithium metal having the surface pattern height less than 50 nm
will form a physical adhesion (anchoring) by pressing with the
separator binder layer formed on the separator fabric surface by
agglomeration of separator binder particles having the size of a
few tens or a few hundreds of nm. The height of the surface pattern
structure more than 1,000 nm is undesirable because the separator
binder layer on the separator surface cannot be densely filled in
the surface pattern structure. When excessive pressing pressure,
temperature and rate are applied to form a physical adhesion,
wrinkling, cracking or separation may occur on the electrode
surface. As described in the following experimental example, as a
result of experiment in manufacturing a 120 nm high grating
structure on the 40 .mu.m thick roll-pressed lithium metal foil
through DVD-R structure stimulation, it can be seen that adhesion
is significantly improved without electrical charge surface
treatment.
[0048] The pattern mold used for nano imprint to fabricate the
lithium metal secondary battery according to the present disclosure
has a raised part or sunken part having the height of
50.about.1,000 nm. The groove may be formed in a reverse shape of
the raised part on the lithium metal foil surface by directly
pressing the raised part onto the lithium metal foil, and the
protrusion may be formed in a reverse shape of the sunken part on
the lithium metal foil surface by pushing the lithium metal foil
into the sunken part. The raised or sunken part may be in the shape
of a pillar, a cone, etc., and preferably, has a tapered shape that
becomes narrower upward the top of the raised part and downward the
bottom of the sunken part because it is easy to insert the
separator into the lithium metal foil surface structure to be
formed later. Accordingly, the raised or sunken part of the pattern
mold is preferably in the shape of a cone, and may have a shape of
a triangular pyramid, a square/rectangular pyramid, a circular
cone, etc., according to the manufacturing method.
[0049] Most preferably, the separator is densely filled in the
lithium metal foil surface structure by shape deformation to form a
perfectly close adhesion with no gap between the lithium metal foil
and the separator. When the lithium metal foil surface structure is
a raised part, the separator has a sunken part that fits into the
raised part, obtaining an engaged cross-sectional structure as if
they are engaged with each other at their corresponding locations,
and likewise, when the lithium metal foil surface structure is a
sunken part, the separator has a raised part that fits into the
sunken part, obtaining an engaged cross-sectional structure as if
they are engaged with each other at their corresponding locations.
In particular, it should be noted that the lithium metal foil
surface structure is intentionally formed by nano imprint, but
shape deformation of the separator is accomplished by the
lamination pressure. For a perfectly close adhesion, it is
necessary to select a proper shape, height and lamination pressure
(considering a lamination load, and an area on which the load acts)
of the surface structure.
[0050] A master mold (a mother mold) of silicon or quartz
manufactured by the etching technique, so-called top-down method
commonly used to manufacture a pattern mold, or an organic mold
manufactured by replicating the master mold may be only
manufactured on a micro scale, and no matter how small the pattern
is, the minimum pattern size (or height) is 10.about.15 .mu.m, and
such molds are unstable for implementing the present disclosure.
The present disclosure proposes using an organic mold manufactured
by replicating a pattern mold manufactured by the bottom-up method
such as self assembly. In particular, it is desirable to use a soft
mold by replication of a pattern mold manufactured by the bottom-up
method using an organic matter of PDMS, ETPTA, polyurethane and
PFPE. With the mold, the surface pattern structure having the
height of 50.about.1,000 nm, i.e., a nano scale micro pattern, may
be formed on the lithium metal foil.
[0051] For example, a method for forming the pattern mold having a
triangular pyramid having the height of 50.about.1,000 nm is
described as below.
[0052] 1) A single layer of silica or polystyrene (PS) particles
having the size of 1 .mu.m or less is formed with a hexagonal
closed pack array (HCP) structure on the surface of a substrate of
glass, a polymer film or a metal foil by self assembly.
[0053] 2) An organic mold is manufactured by PDMS, ETPTA, epoxy or
PFPE replication using the HCP structure single layer surface as a
master mold. After an organic matter is applied to the master mold
and the substrate is removed, the silica or polystyrene particles
may be removed (etched) to form a triangular pyramid nano pattern
in relief or intaglio according to the surface properties (for
example, surface energy) of the material of the organic mold used.
Compared with the use of PDMS or low molecular weight ETPTA, the
use of high molecular weight ETPTA or PFPE can manufacture a hard
mold, and is suitable for nano imprint application.
[0054] 3) Accordingly, the triangular pyramid nano pattern in
different sizes may be formed in relief or intaglio using PFPE on
the lithium metal foil surface by nano imprint. Preferably, the
triangular pyramid nano pattern having the height of 50.about.1,000
nm is formed. Meanwhile, the shape of the sunken part or raised
part of the pattern mold is not necessarily limited to the example
presented above. The pattern may include an island shaped pattern
isolated in four directions from other pattern such as a cone or a
pillar, and a line and space pattern having a repetition of stripe
shaped patterns extending along a direction, spaced apart a
predetermined distance from other pattern. For example, the pattern
may have a repetition of ridges and furrows.
[0055] FIG. 4 is a diagram illustrating an improvement effect when
the lithium metal foil having the surface pattern structure
according to the present disclosure is used as the negative
electrode and adhered to the separator.
[0056] As shown in (a) of FIG. 4, the lithium metal foil negative
electrode 10 having the surface pattern structure 12, the separator
20, the positive electrode 30 are stacked and laminated to form a
monocell 40 as shown in (b). It is obvious that a separator binder
may be applied to the separator 20 as auxiliary adhesion means.
[0057] The positive electrode 30 generally using a PVDF based
binder and an active material of metal oxide having high elastic
modulus forms a good interfacial adhesion with the separator 20. If
the lithium metal foil with the flat and smooth surface is used as
the negative electrode, adhesion with the separator will be poor.
However, because the present disclosure uses the negative electrode
10 having the surface pattern structure 12 on the lithium metal
foil surface, the separator itself and/or the separator binder is
filled in the surface pattern structure 12, and the negative
electrode 10 and the separator 20 are engaged with each other, and
thus a interfacial adhesion between the negative electrode 10 and
the separator 20 is physically improved. Accordingly, even if
stress that may occur in the negative/positive electrode acts as
indicated by the arrow (c), the stress cancels out, thereby
preventing the bending of the monocell 40 and the negative
electrode separation.
EXPERIMENTAL EXAMPLE
[0058] To manufacture an experimental sample according to the
present disclosure, pressing for nano imprint is performed on a
lithium metal foil surface using DVD-R (LG Electronics, R4.7) as a
template. As previously described, it is desirable to manufacture
and use a pattern mold by the bottom-up method, but it can be seen
that pattern transfer can be accomplished using readily available
DVD-R as the pattern mold and its effect is demonstrated.
[0059] FIG. 5 is a photographic image of a process of preparing an
experimental example sample according to the present disclosure.
FIG. 6 is a cross-sectional view of DVD-R used for nano
imprint.
[0060] First, DVD-R (R4.7, LG Electronics) as shown in (a) of FIG.
5 is prepared and dismantled to remove an organic dye and an
aluminum layer to prepare a polycarbonate having a grating
structure (that will be used as a pattern mold). As shown in FIG.
6, DVD-R has the distance dt between track pitches of 740 nm, the
track pitch height h.sub.t of 120 nm, and the track pitch width
w.sub.t of 320 nm. Using the DVD-R as a template, pressing is
performed for 1 min under the pressure of 300 kgf/cm.sup.2 on the
40 .mu.m thick roll-pressed lithium metal foil surface, and as
shown in (b) of FIG. 5, a diffraction phenomenon of DVD-R surface
is also observed on the lithium metal foil surface. This reveals
that the grating structure (including ridges and furrows) of DVD-R
is transferred onto the lithium metal foil surface to form a
pattern on the lithium metal foil surface. Accordingly, it can be
seen that when the pressure is directly applied to the lithium
metal foil surface using the pattern mold as proposed by the
present disclosure, the pattern of the pattern mold can be
transferred onto the lithium metal foil surface.
[0061] FIG. 7 is a photographic image of a general lithium metal
foil with a flat and smooth surface as a comparative example.
[0062] A lithium metal foil (the present disclosure experimental
example) having a grating structure (120 nm height h.sub.t) of
optical disk (DVD-R) on the surface and a general lithium metal
foil (comparative example) as shown in FIG. 7 are prepared, and
each is laminated with a separator for a lithium ion secondary
battery to manufacture an electrode-separator adhesion
structure.
[0063] Each lithium metal foil is 15 mm wide and 50 mm long. For
the lamination, roll-lamination is used, and 10 kgf load is applied
at a rate of 300 mm/sec under 60.degree. C. temperature condition.
For the comparative example, the lamination load of 100 kgf and 250
kgf is prepared.
[0064] An adhesive strength comparison test is performed on the
electrode-separator adhesion structure of the present disclosure
experimental example and the comparative example The adhesive
strength is measured by the commonly used 90.degree. peel-off test,
and the rate is 100 mm /min
[0065] FIG. 8 is a graph showing 90.degree. peel-off test results
of the present disclosure experimental example and the comparative
example.
[0066] Referring to FIG. 8, when the comparative example at the
lamination load of 10 kgf is designated as 100% adhesive strength,
as the lamination load increases to 100 kgf and 250 kgf, the
adhesive strength increases to 185% and 192%. In contrast, in the
case of the present disclosure experimental example, 320% adhesive
strength is achieved at the lamination load of 10 kgf. As described
above, under the same lamination load, the present disclosure
experimental example can have higher adhesive strength 3 times or
more than the comparative example. The high adhesive strength
cannot be achieved even if the lamination load of the comparative
example increases 10 times and 25 times.
[0067] As described above, it can be seen that when the lithium
metal foil having the nano imprint pattern structure on the surface
according to the present disclosure experimental example has a
higher adhesive strength with the separator than the general
lithium metal foil used as the comparative example, and that it is
possible to obtain a good adhesive strength outcome even under low
lamination pressure.
[0068] FIG. 9 is a graph showing 90.degree. peel-off test results
of another experimental example of the present disclosure and a
comparative example.
[0069] Sample manufacturing and testing methods are similar to
those of the experimental example from which the graph of FIG. 8 is
obtained.
[0070] This test evaluates a change in adhesive strength as a
function of a pattern interval on the lithium metal foil surface.
Here, the pattern interval is a distance between patterns, and also
refers to the distance dt between track pitches as previously shown
in FIG. 6.
[0071] The pattern interval is set to 0.00 .mu.m, 0.32 .mu.m, 0.74
.mu.m, and 1.60 .mu.m. The pattern interval of 0.00 .mu.m indicates
the lithium metal foil with no pattern, and may correspond to the
comparative example showing the results of FIG. 8. The pattern
interval of 0.32 .mu.m indicates the lithium metal foil subjected
to pattern transfer using Blu-ray Disk as a template. The pattern
interval of 0.74 .mu.m indicates the lithium metal foil subjected
to pattern transfer using DVD-R as a template as shown in the
experimental example of FIG. 8. The greatest pattern interval of
1.60 .mu.m indicates the lithium metal foil subjected to pattern
transfer using CD-R as a template.
[0072] Each lithium metal foil is 15 mm wide and 50 mm long, for
the lamination, roll-lamination is used, and 10 kgf load is applied
at a rate of 300 mm/sec under 60.degree. C. temperature
condition.
[0073] An adhesive strength comparison test is performed on the
electrode-separator adhesion structure of the present disclosure
experimental example and the comparative example The adhesive
strength is measured by the commonly used 90.degree. peel-off test
and the rate is 100 mm/min
[0074] Referring to FIG. 9, when the comparative example at the
lamination load of 10 kgf is designated as 100% adhesive strength,
the present disclosure experimental example having the pattern
interval of 0.32 .mu.m and 0.74 .mu.m achieves 300% or more
adhesive strength, and the adhesive strength improvement effect is
obviously seen. It is found that the present disclosure
experimental example having the pattern interval of 1.60 .mu.m
achieves about 150% adhesive strength, and has a higher adhesive
strength than the comparative example.
[0075] After an electrode assembly including the
electrode-separator adhesion structure is manufactured, the cell
stiffness of a secondary battery including the electrode assembly
is measured. There is no significant difference between the
presence and absence of pattern. Also, there is no significant
difference in pattern interval difference. This is because the cell
stiffness is dominated by the stiffness of the positive electrode
itself rather than the negative electrode lamination adhesive
strength as expected.
[0076] It is determined whether or not there is a separator
separation at the edge in the electrode-separator adhesion
structure. In the case of the comparative example having the
pattern interval of 0.00 .mu.m, that is, having no pattern, edge
separation is observed. The present disclosure experimental example
having the pattern interval of 0.32 .mu.m and 0.74 .mu.m has no
edge separation. Accordingly, it can be seen that when the pattern
is formed on the lithium metal foil according to the present
disclosure, the adhesive strength with the separator is higher and
the prevention effect of separation at the edge is better. However,
even though the lithium metal foil have the pattern, edge
separation is observed in the sample having the pattern interval of
1.60 .mu.m. Accordingly, in terms of preventing the edge
separation, it is desirable that the pattern interval is not so
great when forming the pattern on the lithium metal foil. For
example, it is desirable that the pattern interval is not greater
than 1.60 .mu.m.
[0077] Hereinafter, a method for fabricating a lithium metal
secondary battery according to the present disclosure will be
described in detail with reference to FIG. 10 based on the
above-described configuration.
[0078] FIG. 10 is a flowchart showing a method for fabricating a
lithium metal secondary battery according to an embodiment of the
present disclosure.
[0079] The method for fabricating a lithium metal secondary battery
according to the present disclosure begins with preparing a lithium
metal foil, and forming a nano imprint pattern structure on a
surface facing a separator to manufacture a negative electrode
(s1). In the case of a one-sided negative electrode, the surface
pattern structure may be formed on one surface of the lithium metal
foil, and in the case of a double-sided negative electrode, the
surface pattern structure may be formed on two sides of the lithium
metal foil. Two one-sided negative electrodes having the surface
pattern structure on one side may be adhered and used as a
double-sided negative electrode.
[0080] The pattern structure is formed by the nano imprint
technique that directly applies the pressure to the surface of the
lithium metal foil using a pattern mold as previously described.
The conditions of the pattern mold for forming the surface pattern
structure having the height of 50.about.1,000 nm, i.e., a nano
scale micro pattern, are described above.
[0081] Subsequently, a separator and a positive electrode are
stacked and laminated on the negative electrode prepared in s1
(s2).
[0082] In this instance, the negative electrode and the separator
are first laminated to manufacture a negative electrode-separator
adhesion structure, then the positive electrode is laminated
together to form an assembly. The negative electrode, the separator
and the positive electrode may be laminated together to form an
assembly. In any case, the lamination load may be 10 kgf. As the
shape of the separator is deformed by the lamination pressure, the
separator is filled in the pattern structure to form a physical
bond between the negative electrode and the separator.
[0083] In the above-described experiment results (FIG. 8), as a
result of experiment in which the grating structure having the
height h.sub.t of 120 nm is formed on the 40 .mu.m thick
roll-pressed lithium metal foil through DVD-R structure
stimulation, it is found that adhesion is greatly increased without
electric charge surface treatment, but if necessary, to further
increase the adhesive strength, methods for improving adhesion may
be also used by applying a separator binder to the lithium metal
foil surface or the separator surface, and by improving the
electric charge properties through corona, RIE and acid treatment
on the lithium metal foil surface or separator surface.
[0084] The cell manufactured in this step may be the monocell 40 of
negative electrode 10-separator 20-positive electrode 30 structure
as shown in (b) of FIG. 4, the A type bicell 140 of positive
electrode 30'-separator 20'-negative electrode 10'-separator
20'-positive electrode 30' structure as shown in (a) of FIG. 11, or
the C type bicell 240 of negative electrode 10'-separator
20'-positive electrode 30'-separator 20'-negative electrode 10'
structure as shown in (b) of FIG. 11. FIG. 11 shows an example in
which the negative electrode 10' is a double-sided negative
electrode, and the positive electrode 30' is a double-sided
positive electrode.
[0085] When the pattern structure is formed on the surface of the
lithium metal foil that will face the separator according to the
present disclosure, it is possible to reduce the conditions for
applying the lamination pressure. It is possible to ensure good
negative electrode-separator adhesive strength under low pressure
as described above with reference to the experiment results of FIG.
8.
[0086] The lithium metal secondary battery according to the present
disclosure may have, as a unit cell, the monocell 40 and the bicell
140, 240 manufactured by the above-described method, and may be
implemented as a stack cell by stacking the unit cells, folding the
unit cell in a zigzag form, and stacking and folding the unit
cells. According to the present disclosure, by virtue of the
improved negative electrode-separator adhesive strength, it is
possible to improve not only the properties of the unit cell itself
but also the properties of the stack using the same.
[0087] FIG. 12 is a diagram illustrating a lithium metal secondary
battery according to still another embodiment of the present
disclosure.
[0088] Referring to FIG. 12, a double-sided negative electrode 10'
and a double-sided positive electrode 30' are prepared, and they
are assembled with a folding separator 20'' folded in zigzag to
manufacture a zigzag folding cell 340. Generally, when zigzag
folding is applied, separation that may occur between the negative
electrode and the folding separator due to an adhesive strength
difference between negative electrode and the positive electrode is
more serious than the problem in the monocell described with
reference to FIG. 1. According to the present disclosure, even
though the lithium metal secondary battery is manufactured in a
zigzag folding form, the improved negative electrode-separator
adhesive strength lowers the separation likelihood, and
significantly reduces the zigzag folding stack defects.
[0089] As described above, the present disclosure manufactures the
negative electrode through a simultaneous nano pattern transfer
process by pre-forming a pattern mold having nano scale protrusion
and recess pattern, and pressing it onto the lithium metal foil
surface. This can form the nano pattern in mass quickly, and is
very suitable for mass production of lithium metal secondary
batteries.
[0090] The surface pattern structure formed by transfer is only
determined by the shape of the pattern mold, and thus there is no
other unexpectable process variable. The bottom-up method can form
a very fine surface pattern structure having the height of
50.about.1,000 nm, and thus a small thickness of 40 .mu.m or less
is desirable in terms of energy density to simultaneously transfer
the pattern onto the lithium metal foil on a suitable scale.
Additionally, basically, the pressing technique is used, and can be
implemented by a very low-priced apparatus.
[0091] Meanwhile, in the battery process, the application of
pressure is necessary in a chronological order in {circle around
(1)} the electrode roll-press (in the present disclosure, the
lithium metal foil is made 20.about.40 .mu.m thick) before
manufacturing a unit cell, {circle around (2)} the unit cell
assembly process ((a) and (b) of FIGS. 4, and s2 in FIG. 10),
{circle around (3)} the stack cell assembly process (for example,
FIG. 11), and {circle around (4)} the jig formation (J/F)
activation process before releasing a final product.
[0092] As opposed to the present disclosure, if a lithium metal
foil with a flat and smooth surface is used, to achieve the unit
cell/stack cell properties, it is necessary to strongly apply the
pressure in the steps {circle around (2)} and {circle around (3)}.
There is no concern about defects that may occur in the
manufactured electrode or separator (in the case of an all solid
state battery, the electrolyte layer) such as cracking, tearing,
warpage and waviness. However, according to the present disclosure,
it is possible to form a high electrode-separator (electrolyte
layer) adhesive interface through the application of low pressure
in the steps of {circle around (2)} and {circle around (3)}.
Accordingly, it is possible to reduce defects that may occur due to
the application of high pressure.
[0093] The method according to the present disclosure is easier and
more economically efficient than technology designed to reduce the
active material composition in the electrode as mentioned in the
related art. Additionally, the method according to the present
disclosure may additionally have advantages as below.
[0094] The pressure application in the steps {circle around (1)},
{circle around (2)} and {circle around (3)} cannot be omitted, but
only the minimum pressure necessary for the process (to prevent the
electrode-separator separation and meander tolerance) may be
applied. This can make it easy to {circle around (4)} remove gas
produced in the J/F activation process.
[0095] Meanwhile, if a very high pressure, temperature and rate is
applied to ensure the properties of the unit cell in the assembly
process, a close adhesion will be formed between the electrode and
the separator by the shape change of the polymer binder such as
PVDF, and a strong electrode-separator adhesive strength will be
achieved. However, this increases the necessary time for the
electrolyte filling during the pre-aging period before the
activation process after the assembly process, resulting in the
increased process cost. According to the present disclosure, the
electrode-separator adhesive strength increases by the application
of the minimum pressure without excessive pressure, thereby
facilitating the electrolyte filling in the pre-aging after
injection.
[0096] While the present disclosure has been hereinabove described
with regard to a limited number of embodiments and drawings, the
present disclosure is not limited thereto and it is obvious to
those skilled in the art that various modifications and changes may
be made thereto within the technical aspects of the present
disclosure and the equivalent scope of the appended claims.
* * * * *